It was established in Overdrive that the Sentinel Knight, in his ghostly and Pre-Zubaan form, could restore powers going as far back as Mighty Morphin' and even traverse time to bring back S.P.D. Red who used to be S.P.D. Blue but who everyone knows as S.P.D. Green. Adding the fact that the Reds had their own during "Forever Red", it suggests that WHEN NECESSARY, anyone who is anyone can fight and then leave just as quickly for inexplicable reasons simply because it takes attention away from the newest team of focus (and because the returning actors are just "Guest Stars").

It was established in Overdrive that the Sentinel Knight, in his ghostly and Pre-Zubaan form, could restore powers going as far back as Mighty Morphin' and even traverse time to bring back S.P.D. Red who used to be S.P.D. Blue but who everyone knows as S.P.D. Green. Adding the fact that the Reds had their own during "Forever Red", it suggests that WHEN NECESSARY, anyone who is anyone can fight and then leave just as quickly for inexplicable reasons simply because it takes attention away from the newest team of focus (and because the returning actors are just "Guest Stars").

Exactly. More or less the Morphing Grid sees something about to happen and lets everyone pull a Dues Ex Machina to save the world/universe/whatever

Now all we have to do is figure out how a giant floating head has a son...

He wasn't always a giant floating head. He just appeared that way in the series because he was stuck in another dimension, or something. They were never really clear about that. But he wasn't always Head-in-a-Jar.